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The structure of a virus

A virus is an infectious particle which can replicate only inside a cell using the synthetic mechanism of the cell. A virus when outside a living cell is usually dormant, and shows no signs of life.Almost two centuries had passed after Antony van Leeuwenhoek had discovered bacteria when Dmitri Ivanovsky’s 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898 took place.Since then thousands of viruses have been described and classified according to their structure and properties.

It is believed that viruses evolved from bacterial plasmids or from bacteria themselves. Viruses are very small compared to bacteria and cannot be usually seen with a compound microscope.An electron microscope which has higher resolution can help in elucidating the structure of a virus. The resolving power of a compound microscope is 200nm(nanometer) and magnification of 2000x, whereas the resolving power of an electron microscope is 50pm(picometer) and magnification up to 10,000,000x.

Structure of a virus

Virus particles (known asvirions) consist of two to three parts

1.The genetic material made from either DNA or RNA.

2.A protein coat that protects these genes called capsid,and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell.Most enveloped viruses are dependent on the envelope for their infectivity.

The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The nucleic acid of a virus can be DNA or RNA, and it can be single stranded, double stranded,linear, circular or segmented, positive sense, negative sense or ambisense(+/-). When the viruses infect a cell,they integrate into the host DNA and replicates using the host machinery and gets released by bursting the cell.The two types of cycle undergone by virus inside a cell lysogenic and lytic cycle.These two cycles can occur in a host but both cannot occur simultaneously.